It has been known that carboxyl groups can be efficiently introduced onto cellulose microfibril surfaces and a homogeneous and clear aqueous cellulose nanofiber solution can be prepared with little energy consumption for microfibrillation by treating a cellulosic material with an inexpensive oxidizing agent sodium hypochlorite in the presence of the radical 2,2,6,6-tetramethyl-1-piperidine-N-oxy (hereinafter referred to as TEMPO) and a bromide or iodide as catalysts (non-patent document 1: Saito, T., et al., Cellulose Commun., 14 (2), 62 (2007)).
This technique for producing cellulose nanofibers is environmentally advantageous as a reaction process in that it uses water as a solvent, the reaction by-product is only sodium chloride, etc., but it should be still improved in the production cost because TEMPO is very expensive. It should also be improved in productivity because the consistency of the cellulosic material during the treatment is as low as about 1% by weight and the treatment period is long (about 1-2 hours).
A TEMPO derivative, 4-hydroxy TEMPO is produced in Japan in a yield of hundreds of tons a year and mainly used as polymerization inhibitors and stain repellents in the field of petrochemical industries because it can be synthesized more easily than TEMPO, it has already been included in safety data sheets under European regulations on chemical substances so that it can be smoothly exported or internally distributed, and it is more biodegradable than TEMPO. Thus, 4-hydroxy TEMPO is much less expensively available than TEMPO. However, it was difficult to convert wood cellulose into nanofibers with 4-hydroxy TEMPO because it could not efficiently introduce carboxyl groups onto microfibril surfaces of wood cellulose.
A possible approach for improving the productivity of cellulose nanofibers is to increase the consistency of the cellulosic material, but a high stirring force is required to homogeneously oxidize high-consistency slurry. However, the application of a strong shearing force during the reaction partially converts cellulose into nanofibers, which cannot be recovered during dehydration/washing steps after the completion of the reaction, resulting in a great loss of the yield. Consequently, a method capable of increasing the reaction speed to shorten the treatment period had to be developed to improve productivity. Moreover, linear polymeric cellulose molecular chains forming nanofibers are partially oxidized and tend to be bent during the TEMPO-catalyzed oxidation treatment as the reaction period increases. As a result, the nanofibers lose their characteristic stiffness and fail to retain their straight fiber shape, thus inviting deterioration in quality such as film strength and barrier properties. For this reason, it would be desirable to shorten the reaction period from the aspect of not only productivity but also quality improvement of cellulose nanofibers. In addition, it was difficult to prepare a homogeneous and clear cellulose nanofiber solution by the conventional TEMPO-catalyzed oxidation treatment from highly crystalline powdery cellulose materials deprived of noncrystalline regions by acid hydrolysis, and a nitroxyl radical having a novel structure ensuring better oxidation performance than TEMPO had to be discovered to convert such highly crystalline cellulosic materials into nanofibers.
Non-patent document 1: Saito, T., et al., Cellulose Commun., 14 (2), 62 (2007).